The weather is the state of the atmosphere with refference to wind, temperature, state of the sea, cloudiness, precipitation, atmospheric pressure, humidity, mist, fog, ice conditions.
It is important for seaman to understand all phenomena conected with the weather and to be able to read weather maps and listen to the weather forecast and report weather conditions at sea such as visibility and the direction of wind and its force according Beaufort Wind Scale.
Winds
Winds are mainly caused by a difference of temperature which in turn is sometimes responsible for the differences of barometric pressure. The strength and speed of wind at any given time depend on the gradient of atmospheric pressure that is the rate at which pressure changes with distance.
Speed of movement of pressure systems
Slowly: Moving at less than 15 knots
Steadily: Moving at 15 to 25 knots
Rather quickly: Moving at 25 to 35 knots
Rapidly: Moving at 35 to 45 knots
Very rapidly: Moving at more than 45 knots
Timing of gale warnings
Imminent: Within 6 hours of time of issue
Soon: Within 6 - 12 hours of time of issue
Later: More than 12 hours from time of issue
Terms referring to wind:
Backing - Indicates the changing of wind in the anticlockwise direction (i.e. from W to SW)
Becoming cyclonic - Indicates that there will be considerable changes in wind direction across the path of a depression within the forecast area.
Wind direction - Indicates the direction from which the wind is blowing.
Veering - Indicates the changing of the wind in a clockwise direction (i.e. from SW to W)
Variable - Indicates the wind constantly changing the direction from which it blows.
Why are ice classes needed?
the Ship Classification Societies "ice class" has a fundamental basis on the safety of the ship hull and the essential propulsion machinery. The class defines sufficient installed power for a safe operation in ice covered waters. The classification also defines certain hull structure against certain level ice, which in the Baltic Sea conditions is defined using the first year ice definition. The classifications also defines the requirements for the propeller shaft as a minimum power for maintaining ship speed in a re-frozen (covered by e.g. brash ice) fairway navigation channel.
When the ice conditions became difficult, traffic restricions are imposed. The restrictions pertain to the availability of icebreaker assistance. Some of the restrictions are about safety independent of assistance standards; some are caused by the availability of icebreaker services. The traffic restrictions are based on Ice Class Rules.